1. Field of the Invention
The present invention relates to power-on reset circuits.
2. Description of the Related Art
Reset terminals are provided in digital integrated circuits (also called simply “ICs” hereinafter). Low-level and high-level reset signals (voltages) are supplied to a reset terminal.
An IC operates while a high-level voltage is being supplied to the reset terminal, for example. The IC is reset when a low-level voltage is supplied to the reset terminal. The IC remains in a reset state while the low-level voltage is being supplied to the reset terminal.
Whether a low-level or high-level voltage is supplied to the reset terminal is determined based on, for example, a power source voltage of the IC. It is possible that the IC will not operate normally in the case where the power source voltage is low. Accordingly, it is preferable to supply the low-level reset signal to the reset terminal and keep the IC in a reset state. However, in the case where the power source voltage is not low, it is preferable to supply the high-level reset signal and cause the IC to operate. Note that it is preferable to reset the IC using the reset signal when the supply of the power source voltage starts and the IC begins to operate.
A power-on reset circuit is known as a circuit for supplying a reset signal to a reset terminal based on a power source voltage. For example, Japanese Unexamined Patent Application Publication No. 2003-8426 discloses a power-on reset circuit that generates a reset signal when a power source voltage is applied and resets a flip-flop circuit provided in the IC using the reset signal. This type of power-on reset circuit includes active elements such as inverters and the like.
The power-on reset circuit requires the power source voltage in order to operate the active elements such as inverters. A configuration in which the same voltage source is used as a voltage source for supplying a power source voltage to the power-on reset circuit and a voltage source for supplying the power source voltage to the IC can be considered as one way of reducing the size, cost, and so on of the device as a whole. Japanese Unexamined Patent Application Publication No. 2003-8426, however, does not teach such a configuration.
In the case where the same voltage source is used as the voltage source for the power-on reset circuit and the voltage source for the IC, if the voltage of the voltage source (the power source voltage) drops, the voltage required to operate the inverter in the power-on reset circuit, for example, cannot be obtained. There is thus a risk that the power-on reset circuit will no longer operate normally.